To reduce emissions in passenger vehicles equipped with Otto engines, 3-way catalytic converters, which convert exhaust gas to a sufficient degree only if the λ air-fuel ratio is adjusted with high precision, are routinely used as exhaust gas purification systems. For this purpose, the λ air-fuel ratio is measured with the aid of an exhaust-gas sensor situated upstream from the exhaust-gas purification system. The oxygen storage capacity of such an exhaust-gas purification system is utilized for receiving oxygen in lean phases and for releasing it again in rich phases. This allows a conversion of oxidizable pollutant gas components of the exhaust gas. An exhaust-gas sensor postconnected to the exhaust gas purification system monitors the oxygen storage capacity of the exhaust gas purification system. The oxygen storage capacity must be monitored within the framework of the on-board diagnosis (OBD), since it represents a measure of the conversion ability of the exhaust gas purification system.
To determine the oxygen storage capability, either oxygen is applied to the exhaust gas purification system in a lean phase and then discharged in a rich phase having a known lambda value of the exhaust gas, taking the passing exhaust gas quantity into account, or the oxygen in the exhaust gas purification system is first evacuated in a rich phase and then replenished in a lean phase having a known lambda value of the exhaust gas, taking the passing exhaust gas quantity into account. The lean phase is terminated when the exhaust gas sensor postconnected to the exhaust gas purification system detects that the exhaust gas purification system is no longer able to store the oxygen. In the same way, a rich phase is terminated when the exhaust gas sensor detects the passage of rich exhaust gas. The oxygen storage capacity of the exhaust gas purification system corresponds to the quantity of reducing agents supplied for the evacuation during the rich phase or the oxygen quantity supplied for the replenishment during the lean phase. The precise quantities are calculated from the signal of the upstream exhaust gas sensor and from the exhaust gas mass flow determined from other sensor signals.
If the dynamic response of the upstream exhaust gas sensor diminishes, e.g., as a result of contamination or aging, then the air-fuel ratio is no longer able to be adjusted with the required precision, so that the conversion power of the exhaust gas purification system drops. Furthermore, there may be deviations in the diagnosis of the exhaust gas purification system, which may cause an actually properly functioning exhaust gas purification system to be erroneously classified as non-functional. The law requires a diagnosis of the sensor characteristics during a vehicle operation in order to ensure that the prescribed fuel-air ratio continues to be adjustable with sufficient precision, so that the emissions do not exceed the allowed limit values and the exhaust gas purification system is monitored correctly. The OBdII stipulations require monitoring of lambda sensors and other exhaust gas sensors not only with regard to their electrical functioning, but also with regard to their response behavior, i.e., a worsening of the sensor dynamics, which may manifest itself in a greater time constant and/or dead time, must be detected. Dead times and delay times between a change in the exhaust gas composition and its detection requite an on-board check so as to ascertain whether they are still allowable for the usage functions, i.e., for control, regulation and monitoring functions that utilize the sensor signal. Typically, the dead time from a mixture change to the signal flank and a particular rise time, e.g., from 0% to 63% or from 30% to 60% of the signal travel, are used as characteristic quantities for the dynamic characteristics of exhaust gas sensors. The dead time also includes the gas propagation time from the engine outlet to the sensor, and therefore changes in a manipulation of the sensor installation location, in particular.
In Diesel engines, broadband lambda sensors are used as gas sensors or as gas concentration sensors, and in connection with SCR catalysts, NOx sensors are used as well. The latter additionally also supply an O2 signal. The O2 signal of the broadband lambda sensor or the NOx sensor is used not only for operating exhaust gas aftertreatment systems in the Diesel engine, but also for the motor-internal emission reduction. The measured O2 concentration in the exhaust gas or the measured lambda signal is used to dynamically adjust the air-fuel mixture in a precise manner and to thereby minimize the deviations in the untreated emissions. In Diesel engines having a NOx storage catalyst (NST), a separate broadband lambda sensor is needed upstream and downstream from the catalyst in each case in order to reliably represent the rich operation required for the regeneration. An engine-internal emission reduction and an NSC operation also pose certain minimum requirements on the dynamic characteristics of the O2 sensor. Nowadays, the rise time of the O2 signal is monitored during the transition from load to trailing throttle, i.e., in a rise from a certain percentage below the normal O2 content of air, to 21%. If the sensor signal even fails to reach a certain intermediate value after a maximum time, then this will be interpreted as a dead time error. In Diesel engines having a NOx storage catalyst (NSC), the response behavior of the lambda sensors upstream and downstream from the catalyst is usually compared as well.
For future vehicle generations or model years, it can be expected that monitoring of the sensor dynamics at a dropping O2 concentration will be required as well. In addition, there will be no more trailing throttle phases in hybrid vehicles in the future and thus no phases featuring a constant O2 concentration of 21%. Initial approaches for these supplementary requirements are the active monitoring in the German publication DE 10 2008 001 121 A1 and the observer-based method in DE 10 2008 040 737 A1.
A method for monitoring dynamic characteristics of a broadband lambda sensor is discussed in DE 10 2008 040 737 A1; here, a measured lambda signal, which corresponds to an oxygen concentration in the exhaust gas of an internal combustion engine, is determined with the aid of the broadband lambda sensor, and an observer, which uses the input variables to generate a modeled lambda signal, is assigned to the internal combustion engine, an estimation error signal being generated on the basis of the difference between the modeled lambda signal and the measured lambda signal, or from the difference between a signal derived from the modeled lambda signal and a signal derived from the measured lambda signal. In this context, it is provided that a measure of the dynamic characteristics of the broadband lambda sensor, characterized by a dead time and a response time, is ascertained from an analysis of the estimation error signal or a variable derived therefrom, and that the measure of the dynamic characteristics is compared with predefined limit values in order to evaluate the extent to which the dynamic characteristics of the broadband lambda sensor suffice for an intended operation of the internal combustion engine.
In addition, a method and a device for the online adaptation of an LSU dynamic model is discussed in DE 10 2008 001 569 A1. More specifically, the document relates to a method and a device for adapting a dynamic model of an exhaust gas sensor, which is part of an exhaust gas tract of an internal combustion engine and used to ascertain a lambda value for controlling an air-fuel composition; in parallel therewith, a simulated lambda value is calculated in a control device or in a diagnostic device of the internal combustion engine, and a user function uses both the simulated and the measured lambda value. A step behavior of the exhaust gas sensor is determined in this context, by analyzing a signal change in an excitation of the system during an ongoing vehicle operation, and the dynamic model of the exhaust gas sensor is adapted on the basis of these results of the dynamic model.
Functions for the dynamic monitoring of broadband lambda sensors are utilized to identify the sensor characteristics. Similar requirements as for O2 signals or O2 sensors exist for other gas concentration signals of exhaust gas sensors, e.g., a NOx signal. Similarities between the monitoring functions must therefore be assumed.
The method according to DE 10 2008 001 121 A1 relates to active monitoring. It includes an excitation by a test injection, which increases both the fuel consumption and emissions. While the method according to DE 10 2008 040 737 A1 is operating passively, it assumes a so-called observer, whose application is costly. In addition, both methods primarily focus on the detection of more substantial dead time changes.
In an application by the applicant, which is German patent document 10 2011 088 296.0, filed on Dec. 12, 2011, another method and device are described for implementing the method for dynamic monitoring of gas sensors of an internal combustion engine, which, for instance, are disposed as exhaust-gas sensors in the exhaust gas tract of an internal combustion engine as part of an exhaust gas monitoring or reducing system or as gas concentration sensors in an air supply of the internal combustion engine. Depending on the geometry, measuring principle, aging or contamination, the gas sensors exhibit a low pass behavior, and a dynamic diagnosis is performed in response to a change in the gas state variable to be detected based on a comparison of a modeled and a measured signal, the measured signal being an actual value of an output signal of the gas sensor, and the modeled signal being a model value. The output signal of the gas sensor is filtered by a high-pass filter, and higher frequency signal components are analyzed when a change occurs in the gas state variable to be measured, such as also a gas concentration.
A change may occur as a result of an excitation of the internal combustion engine. This method makes it possible to detect and quantify changes with regard to the dynamic response in gas sensors. Gas sensors within the meaning of the present invention are sensors which are able to measure the states of a gas and detect changes therein. The state of the gas may be described by a temperature of the gas, a gas pressure, a mass flow rate of a gas and/or a concentration of a particular gas component, e.g., an oxygen component or a NOx component. Gas sensors have a typical low pass behavior which depends on the geometry of their configuration, for instance. In addition, such sensors may change their response behavior due to aging or external influences (e.g., due to sooting in Diesel engines).
This dynamic diagnosis method is basically suitable for monitoring or identifying a T63 filter time constant of sensors in the air and exhaust gas system of internal combustion engines. To do so, the functionality compares the signal energies of the sensor signal and a model-based reference signal in the range of higher frequencies. In principle, however, it is possible that a slow but electrically oscillating sensor could erroneously be detected as dynamically properly functioning when using the monitoring principle according to German patent document 10 2011 088 296.0, filed on Dec. 12, 2011. For example, this scenario is conceivable when an exhaust gas sensor is heavily contaminated with carbon particulate, but electromagnetic interference is coupled into the cable harness or the evaluation circuit has an electrical fault (double error).